Many of printed materials had been printed using a printing plate like offset printing, however, with recent trend toward better pictures and speeding-up of electrophotography and ink-jet printing, printing traders which use a electrophotographic printer or an ink-jet printer for a relatively small volume of printing have been increased in number.
Such an electrophotographic printer is provided with a plurality of paper feed sources usually loaded with printing paper, especially, different sizes of printing paper. Therefore, by selecting a paper feed source in accordance with a paper size to be printed, printing can be printed on different sizes of paper without need for replacing stacked paper.
In executing printing jobs having a large number of print copies or pages, the same size of paper sheets used for printing are respectively stacked on a plurality of paper feed sources in advance. If no paper sheets exist on the paper feed source which first starts paper feeding, a paper feed source is shifted to the next paper feed source to continue printing. Thus, even if printing is required by more than the number of paper sheets stackable on one paper feed source, continuous printing can be performed without any interruption of printing due to paper supply.
As disclosed in the following Patent Document 1, for example, if it is detected that no paper sheets exist on a paper feed source during continuous printing, printing operation is interrupted, paper sheet sizes stacked on all paper feed sources provided in a copying machine are detected, and the paper feed source stacking the paper sheets of the size being used before the interruption is searched, thus enabling restart paper feeding with the searched paper feed source. [Patent Document 1] Japanese Patent Laid-Open No. 9-301561
In a prior art as disclosed in Patent Document 1, however, when there is a printing job having a great many print copies and shifting of a paper feed source occurs, paper feeding is not performed from the next paper feed source until the first paper feed source has no paper sheets thereon, therefore only after all the paper sheets on the first paper feed source has been completely fed, it cannot be confirmed whether or not the same paper sheets are stacked on the next paper feed source and whether or not paper jam will occur.
For example, in the case where a printer equipped with three paper feed sources, each of which is capable of stacking 1,000 paper sheets, executes a printing job of 2,500 paper copies, the printer feeds the first 1000 paper sheets from the first paper feed source and shifts the first paper feed to the second one to continue printing. However, the printer cannot determine whether or not the paper sheets are of the same size as that before the shifting until the shifting has been completed.
If the paper sheets are not of the same size, the printer must discontinue the printing operation and, even if stacking 2,500 paper sheets or more, the printer has a possibility of discontinuing printing operation in the course of the printing operation.
If the paper sheets are different from the paper sheets to be expected immediately after the printing job is commenced, there is a high possibility of an operator standing nearby an image forming apparatus, and the operator can replace the paper sheets, so that there are few cases where execution of the printing job is interrupted over a long time.
However, if the paper feed source is shifted after a printing job is started, no paper sheets are on a particular paper feed source (for example, after 1,000 print copies are printed) and paper sheets on the working paper feed source after the shifting are different from desired paper sheets, there is a low possibility of an operator standing nearby the image forming apparatus because the printing job is under execution, so that there is a possibility of the execution of the printing job being interrupted over a long time.
Paper sheets are generally apt to have paper jam most frequently immediately when the paper sheets have been first fed from a paper feed source after having been stacked. Therefore, if no paper jam occurs at the first paper feeding, there is a low possibility of occurrence of the subsequent paper jam. Therefore, there if a high possibility of occurrence of paper jam when the first paper feed source is shifted to the second paper feed source.
In the event of occurrence of such a condition, printing stops, therefore, even if three paper feed sources capable of stacking a total of 3,000 paper sheets are provided, it is necessary to check whether or not the paper sheets are properly printed continuously even after a paper feed source is shifted in the course of printing.